The invention concerns a process for simultaneous drying and crystallization of stranded crystallizable thermoplastic such as polyethylene terephthalate, in which the plastic is extruded from the melt as strands, solidified at the surface by quenching in water, crystallized by treatment with gas, and dried, using a mechanism for cooling and drying of strands leaving the dies having a discharge chute with its intake end under the dies and a system producing a flow of coolant liquid within a quenching section, with a following dewatering section in which the discharge chute has openings for the free passage of the coolant liquid, with an attached drying section in which the discharge chute has closely spaced nozzles for passage of gas, and with a granulator following the drying section.
Such a process, in which molten polyester strands are first quenched in water, then granulated, and finally crystallized, as the granulation, in a container with heating at a temperature of greater than 130.degree. C. for 2 to 30 minutes, is known from German Laid-Open Patent 19 05 677. A further development of this process is also known from German Laid-Open Patent 21 40 265. In that process, the plastic ribbons are first quenched in a water bath and are then carried through a housing, the interior of which is heated by hot gas that is blown in. The treatment with the hot gas lasts for 2 to 3 minutes, with a gas temperature of 110.degree. to 250.degree. C.
The invention is based on the objective of designing the process step of drying and crystallization in a well-known and practically proven mechanism for cooling and drying of strands of crystallizable thermoplastic leaving the dies, so as to give short processing times. It is intended that the equipment which is known from German Patent 39 00 250 C2, as shown particularly in FIG. 2 of this patent, be used.
The objective has been achieved with this patent in such a manner that the lengths of the quenching section, the dewatering section, and the drying section have been selected, considering the rate of flow of the strands along the discharge chute, so as to give a processing time of not more than 1.5 seconds in the quenching section to produce a strand surface temperature of at least 100.degree. C., not more than about 0.1 second in the dewatering section, and not more than about 20 seconds for crystallization in the drying section, with the surface temperature at the end of the quenching section being generally maintained.
The process carrried out using the known mechanism, the lengths of which must be properly adjusted for this purpose, brings the surprising result that the time for crystallization, during which the strands are held at a temperature of at least 100.degree. C. following their quenching, can be considerably reduced in comparison with the known process. Specifically, the crystallization time can be reduced to the processing time in the drying section, which is not greater than about 20 seconds. This surprising result appears because of the flow of temperature-controlled air over the strands so that they can be kept at their surface temperature, i.e., at least 100.degree. C., over practically the entire length of the drying section. As a result, there is no residual surface water which could interfere with the crystallization. In the known equipment, there is a substantial quantity of surface water because there is no intensive dewatering of the strands or granules before the region in which crystallization occurs. The water must first be evaporated in the crystallization region, removing so much heat from the material that the crystallization process is substantially slowed. This loss of heat must be compensated by adding heat to the region involved. That does not occur in the process according to the invention, as in this case the dewatering section assures that the strands arrive at the drying section practically free of water. If there is still surface water on the strands when they arrive at the drying section, it is completely removed at the very beginning, so that the drying section can produce its action on crystallization practically completely free of the harmful effect of residual water. There is a further advantage in that the process of the invention can be operated in equipment of relatively simple design which has been proven in practice. That substantially reduces the equipment cost.
Dewatering in the dewatering section is particularly intensive if the strands are exposed to an air flow to dry them after they leave the cooling liquid.
It is convenient to chop the strands into granules in a granulator connected to the drying section. The granules, which are dry when produced, can then be held longer at the surface temperature of the strands in the vicinity of the drying section without any problems occurring due to evaporation of residual water. Thus the crystallization process can be continued economically.